The present invention relates to implantable living tissue stimulators, such as neural stimulators or cochlear stimulators, that do not use output coupling capacitors. More particularly, the invention relates to circuit designs and methods for use within the output stage of such stimulators that prevent the flow of dc current between paired electrodes.
Most implantable living tissue stimulators employ coupling capacitors to provide dc isolation between the output stage of such devices and the tissue-stimulating electrodes. A dc current flow through living tissue can be very undesirable, particularly when allowed to continue over prolonged periods of time. This is because prolonged dc current flow through living tissue can cause tissue growth at one of the electrodes, and tissue destruction at the other electrode, and can also cause excessive repeated firing of muscle neuron tissue. Hence, except for certain tissue growth and tissue healing devices, coupling capacitors are normally used to block all dc current flow. Such capacitors advantageously allow an ac current to flow through and stimulate the living tissue, such as a biphasic current pulse, but prevent any undesirable dc current flow in the tissue. More particularly, such capacitors limit the coulombs that are allowed to flow in one direction (where coulomb flow per unit time in one direction is the electrical definition of dc current).
Unfortunately, the output coupling capacitors used within a living tissue stimulator may represent a significant portion of the total volume of the device, ofttimes occupying much more volume than the integrated circuit chip which contains all of the stimulating circuitry. This is particularly the case when the stimulator is a multichannel stimulator, employing e.g., 16 electrodes, and hence at least 16 coupling capacitors, as is common with implantable cochlear stimulators. In order to reduce the overall volume and size of a living-tissue stimulator device, it would therefore be desirable to avoid the use of output coupling capacitors. However, if coupling capacitors are not used, then there is a need for other means associated with the implant device to assure that no dc current flows through the electrodes.
The present invention addresses the above and other needs.